The swapped sim issue for femtocells

One of many minor issues when managing a femtocell that can be easily overlooked relates to SIM cards and local access lists. This adds some complexity into managing the local whitelist, especially if you lose your phone.

Femtocells don’t know anything about mobile telephone numbers

Mobile phones and their networks communicate using the SIM card number, known as IMSI (International Mobile Subscriber Identity, effectively the unique number of the SIM card inside your phone or mobile device. All signalling and communication messages relating to your mobile device are based on the IMSI, not the mobile phone number.

When calling another phone on the same network, the number you call is converted into an IMSI and used to connect to it by looking up an online database.

The femtocell doesn’t store phone numbers in its whitelist

In order to prevent abuse or misuse of femtocells, many operators provide a whitelist for each one. Customers can typically register up to 30 or more mobile telephone numbers, often through a self-service webpage, which is then used to restrict access. The whitelist and updates are downloaded to the femtocell, usually almost instantaneously, and used to determine whether to accept or reject attach requests from passing mobile devices.

Femtocell whitelists need to be validated before they are used

During this update procedure, it is important to check that the mobile phone numbers relate to subscribers on the same mobile network. With the advent of mobile number portability, you can no longer determine which network owns any specific number just by looking at it. This check is typically done by looking up the data on the customer care database.

What happens if you lose you phone or it’s stolen

The benefit of associating a SIM card with your phone number in a database is that it can be changed. If your phone is stolen, breaks (maybe you dropped it in a puddle) or the SIM card is faulty, you can be sent a replacement. The network operator can update their database to map your phone number to the new SIM card, and everything then works as before.

But this also impacts your femtocell whitelist

One issue that can be overlooked is that where a SIM is swapped, then the whitelist held inside the femtocell must also be updated. This requires a process to automatically replace the old IMSI with the new IMSI number and push this down into the femtocell. Ideally, this would work transparently as part of the existing SIM swap process.

Another similar update should happen if any of your whitelist numbers decides to move to another mobile operator. They would get a new SIM card and the old one would be switched off. The phone number should be deleted from the whitelist both on the self-service page and inside the femtocell.

There are a number of unexpected and hidden back-office impacts of femtocells

Operators have complex and extensive range of back-office systems which handle huge numbers of customers and transactions every day. Even relatively unusual events occur in such high numbers. This means it’s important to identify these issues and automate solutions where possible before scaling up to full production. The extensive trials and market testing of femtocells has uncovered and resolved most of these issues for some very large network operators.